Internet Engineering Task Force (IETF) M. Bjorklund
Request for Comments: 8340 Tail-f Systems
BCP: 215 L. Berger, Ed.
Category: Best Current Practice LabN Consulting, L.L.C.
ISSN: 2070-1721 March 2018
YANG Tree Diagrams
Abstract
This document captures the current syntax used in YANG module tree
diagrams. The purpose of this document is to provide a single
location for this definition. This syntax may be updated from time
to time based on the evolution of the YANG language.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8340.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Bjorklund & Berger Best Current Practice [Page 1]

RFC 8340 YANG Tree Diagrams March 2018
Table of Contents
1. Introduction ....................................................22. Tree Diagram Syntax .............................................32.1. Submodules .................................................52.2. Groupings ..................................................52.3. yang-data ..................................................52.4. Collapsed Node Representation ..............................62.5. Comments ...................................................62.6. Node Representation ........................................63. Usage Guidelines for RFCs .......................................73.1. Wrapping Long Lines ........................................83.2. Groupings ..................................................83.3. Long Diagrams ..............................................84. YANG Schema Mount Tree Diagrams .................................94.1. Representation of Mounted Schema Trees ....................105. IANA Considerations ............................................126. Security Considerations ........................................127. Informative References .........................................12
Authors' Addresses ................................................131. Introduction
YANG tree diagrams were first published in RFC 6536. Such diagrams
are used to provide a simplified graphical representation of a data
model and can be automatically generated via tools such as "pyang"
[PYANG]. This document describes the syntax used in YANG tree
diagrams. It is expected that this document will be updated or
replaced as changes to the YANG language [RFC7950] necessitate.
Today's common practice is to include the definition of the syntax
used to represent a YANG module in every document that provides a
tree diagram. This practice has several disadvantages; therefore,
the purpose of this document is to provide a single location for this
definition. It is not the intent of this document to restrict future
changes, but rather to ensure that such changes are easily identified
and suitably agreed upon.
An example tree diagram can be found in Section 3 of [RFC8343]; the
following is a portion of it:
+--rw interfaces
+--rw interface* [name]
+--rw name string
+--rw description? string
+--rw type identityref
+--rw enabled? boolean
+--rw link-up-down-trap-enable? enumeration {if-mib}?
Bjorklund & Berger Best Current Practice [Page 2]

RFC 8340 YANG Tree Diagrams March 20182. Tree Diagram Syntax
This section describes the meaning of the symbols used in YANG tree
diagrams.
A full tree diagram of a module represents all elements. It includes
the name of the module and sections for top-level module statements
(typically containers), augmentations, rpcs, and notifications all
identified under a module statement. Module trees may be included in
a document as a whole, by one or more sections, or even by subsets of
nodes.
A module is identified by "module:" followed by the module-name.
This is followed by one or more sections, in order:
1. The top-level data nodes defined in the module, offset by
two spaces.
2. Augmentations, offset by two spaces and identified by the keyword
"augment" followed by the augment target node and a colon (":")
character.
3. RPCs, offset by two spaces and identified by "rpcs:".
4. Notifications, offset by two spaces and identified by
"notifications:".
5. Groupings, offset by two spaces and identified by the keyword
"grouping" followed by the name of the grouping and a colon (":")
character.
6. yang-data, offset by two spaces and identified by the keyword
"yang-data" followed by the name of the yang-data structure and a
colon (":") character.
The relative organization of each section is provided using a
text-based format that is typical of a file system directory tree
display command. Each node in the tree is prefaced with "+--".
Schema nodes that are children of another node are offset from the
parent by three spaces. Sibling schema nodes are listed with the
same space offset and, when separated by lines, are linked via a
vertical bar ("|") character.
Bjorklund & Berger Best Current Practice [Page 3]

RFC 8340 YANG Tree Diagrams March 20182.1. Submodules
Submodules are represented in the same fashion as modules but are
identified by "submodule:" followed by the (sub)module-name. For
example:
submodule: <module-name>
+--<node>
| +--<node>
| +--<node>
2.2. Groupings
Nodes within a used grouping are normally expanded as if the nodes
were defined at the location of the "uses" statement. However, it is
also possible to not expand the "uses" statement but to instead print
the name of the grouping.
For example, the following diagram shows the "tls-transport" grouping
from [RFC7407] unexpanded:
+--rw tls
+---u tls-transport
If the grouping is expanded, it could be printed as:
+--rw tls
+--rw port? inet:port-number
+--rw client-fingerprint? x509c2n:tls-fingerprint
+--rw server-fingerprint? x509c2n:tls-fingerprint
+--rw server-identity? snmp:admin-string
Groupings may optionally be present in the "groupings" section.
2.3. yang-data
If the module defines a "yang-data" structure [RFC8040], these
structures may optionally be present in the "yang-data" section.
Bjorklund & Berger Best Current Practice [Page 5]

RFC 8340 YANG Tree Diagrams March 20182.4. Collapsed Node Representation
At times when the composition of the nodes within a module schema is
not important in the context of the presented tree, sibling nodes and
their children can be collapsed using the notation "..." in place of
the text lines used to represent the summarized nodes. For example:
+--<node>
| ...
+--<node>
+--<node>
+--<node>
2.5. Comments
Single line comments, starting with "//" (possibly indented) and
ending at the end of the line, may be used in the tree notation.
2.6. Node Representation
Each node in a YANG module is printed as:
<status>--<flags> <name><opts> <type> <if-features>
<status> is one of:
+ for current
x for deprecated
o for obsolete
<flags> is one of:
rw for configuration data nodes and choice nodes
ro for non-configuration data nodes and choice nodes,
output parameters to rpcs and actions, and
notification parameters
-w for input parameters to rpcs and actions
-u for uses of a grouping
-x for rpcs and actions
-n for notifications
mp for nodes containing a "mount-point" extension statement
Case nodes do not have any <flags>.
Bjorklund & Berger Best Current Practice [Page 6]

RFC 8340 YANG Tree Diagrams March 2018
<name> is the name of the node
(<name>) means that the node is a choice node
:(<name>) means that the node is a case node
If the node is augmented into the tree from another module,
its name is printed as <prefix>:<name>, where <prefix> is the
prefix defined in the module where the node is defined.
If the node is a case node, there is no space before the
<name>.
<opts> is one of:
? for an optional leaf, choice, anydata, or anyxml
! for a presence container
* for a leaf-list or list
[<keys>] for a list's keys
/ for a top-level data node in a mounted module
@ for a top-level data node of a module identified in a
mount point parent reference
<type> is the name of the type for leafs and leaf-lists
If the type is a leafref, the type is printed as either
(1) "-> TARGET", where TARGET is the leafref path,
with prefixes removed if possible or (2) "leafref".
<if-features> is the list of features this node depends on,
printed within curly brackets and a question mark "{...}?"
Arbitrary whitespace is allowed between any of the whitespace-
separated fields (e.g., <opts> and <type>). Additional whitespace
may, for example, be used to "column align" fields (e.g., within a
list or container) to improve readability.
3. Usage Guidelines for RFCs
This section provides general guidelines related to the use of tree
diagrams in RFCs.
Bjorklund & Berger Best Current Practice [Page 7]

RFC 8340 YANG Tree Diagrams March 20183.1. Wrapping Long Lines
Internet-Drafts and RFCs limit the number of characters that may
appear in a line of text to 72 characters. When the tree
representation of a node results in a line being longer than this
limit, the line should be broken between <opts> and <type> or between
<type> and <if-feature>. The new line should be indented so that it
starts below <name> with a whitespace offset of at least two
characters. For example:
notifications:
+---n yang-library-change
+--ro module-set-id
-> /modules-state/module-set-id
Long paths (e.g., leafref paths or augment targets) can be split and
printed on more than one line. For example:
augment /nat:nat/nat:instances/nat:instance/nat:mapping-table
/nat:mapping-entry:
The previously mentioned "pyang" command can be helpful in producing
such output; for example, the notification diagram above was produced
using:
pyang -f tree --tree-line-length 50 ietf-yang-library.yang
When a tree diagram is included as a figure in an Internet-Draft or
RFC, "--tree-line-length 69" works well.
3.2. Groupings
If the YANG module is comprised of groupings only, then the tree
diagram should contain the groupings. The "pyang" compiler can be
used to produce a tree diagram with groupings using the
"-f tree --tree-print-groupings" command-line parameters.
3.3. Long Diagrams
Tree diagrams can be split into sections to correspond to document
structure. As tree diagrams are intended to provide a simplified
view of a module, diagrams longer than a page should generally be
avoided. If the complete tree diagram for a module becomes too long,
the diagram can be split into several smaller diagrams. For example,
it might be possible to have one diagram with the data node and
another with all notifications. If the data nodes tree is too long,
it is also possible to split the diagram into smaller diagrams for
Bjorklund & Berger Best Current Practice [Page 8]

RFC 8340 YANG Tree Diagrams March 2018
different subtrees. When long diagrams are included in a document,
authors should consider whether to include the long diagram in the
main body of the document or in an appendix.
An example of such a split can be found in [RFC7407], where
Section 2.4 of that document shows the diagram for "engine
configuration":
+--rw snmp
+--rw engine
// more parameters from the "engine" subtree here
Further, Section 2.5 of [RFC7407] shows the diagram for "target
configuration":
+--rw snmp
+--rw target* [name]
// more parameters from the "target" subtree here
The previously mentioned "pyang" command can be helpful in producing
such output; for example, the above example was produced using:
pyang -f tree --tree-path /snmp/target ietf-snmp.yang
4. YANG Schema Mount Tree Diagrams
"YANG schema mount" is defined in [SCHEMA-MOUNT] and warrants some
specific discussion. Schema mount is a generic mechanism that allows
for the mounting of one or more YANG modules at a specified location
of another (parent) schema. The specific location is referred to as
a "mount point", and any container or list node in a schema may serve
as a mount point. Mount points are identified via the inclusion of
the "mount-point" extension statement as a substatement under a
container or list node. Mount point nodes are thus directly
identified in a module schema definition and can be identified in a
tree diagram as indicated above using the "mp" flag.
Bjorklund & Berger Best Current Practice [Page 9]

RFC 8340 YANG Tree Diagrams March 2018
In the following example taken from [YANG-NIs], "vrf-root" is a
container that includes the "mount-point" extension statement as part
of its definition:
module: ietf-network-instance
+--rw network-instances
+--rw network-instance* [name]
+--rw name string
+--rw enabled? boolean
+--rw description? string
+--rw (ni-type)?
+--rw (root-type)
+--:(vrf-root)
| +--mp vrf-root
4.1. Representation of Mounted Schema Trees
The actual modules made available under a mount point are controlled
by a server and are provided to clients. This information is
typically provided via the schema mount module
("ietf-yang-schema-mount") defined in [SCHEMA-MOUNT]. The schema
mount module supports the exposure of both mounted schema and
"parent-references". Parent references are used for XML Path
Language (XPath) evaluation within mounted modules and do not
represent client-accessible paths; the referenced information is
available to clients via the parent schema. Schema mount also
defines an "inline" type of mount point, where mounted modules are
exposed via the YANG library module.
Although the modules made available under a mount point are not
specified in YANG modules that include mount points, the document
defining the module will describe the intended use of the module and
may identify both modules that will be mounted and parent modules
that can be referenced by mounted modules. An example of such a
description can be found in [YANG-NIs]. A specific implementation of
a module containing mount points will also support a specific list of
mounted and referenced modules. In describing both intended use and
actual implementations, it is helpful to show how mounted modules
would be instantiated and referenced under a mount point using tree
diagrams.
In such diagrams, the mount point should be treated much like a
container that uses a grouping. The flags should also be set based
on the "config" leaf mentioned above, and the mount-related options
indicated above should be shown for the top-level nodes in a mounted
or referenced module. The following example, taken from [YANG-NIs],
Bjorklund & Berger Best Current Practice [Page 10]